Anthony Marra exploring the erotic inner life of Downton Abbey’s upstanding Mr. Bates, Elizabeth Fremantle writing from the perspective of Anne Boleyn’s dog, Brad Meltzer and A.J. Jacobs co-writing a story whose characters argue about the last words they’ll utter, and Gabrielle Zevin impersonating a former best-selling author struggling to figure out Twitter—these are some of the leading authors joining in the second #TwitterFiction Festival, March 12–16.